Mobile oven design Buoy...HELP

[dmun]

Re: Mobile oven design Buoy...HELP

mobile and brick domes do not get along

This is correct. Most successful mobile ovens for catering use modular ovens: fewer pieces means fewer problems.

FB insulation board between the fire brick and the steel deck. The insulation board MSDS states that it will reduce 800F to 170F

This too is correct. My concern is that your brick floor is butting up against the steel ring without any insulation between the two. Heat travels sideways, not just up and down.

Tack weld the dome back into place at a few joints so as to keep it in place but also reduce its ability to transfer excess heat into the trailer directly through the steel parts.

I think any metal to metal contact is going to kill you as far as heat transfer.

Your oven, no matter what it's made from needs to completely isolated from your metal support structure. Think about your oven enveloped in insulation, with no direct contact. safety issues aside, you're going to have to haul a second trailer for firewood.

[P712]

Re: Mobile oven design Buoy...HELP

Thanks guys for the comments, it helps keep me thinking about new ways to make this work.

My initial plan was to lay down the FB insulation board between the fire brick and the steel deck. The insulation board MSDS states that it will reduce 800F to 170F. If this is not the case I assume I'll have a serious problem.

As far as the dome is concerned, I thought seriously about a traditional brick dome for a long time but everyone I asked in the Alan Scott camp advised that mobile and brick domes do not get along. The bouncing and bumps of the trailer end up causing serious damage the the dome. I do not want to deal with this. We will be using this oven as a tool for a professional catering company. So my plan is to finish the deck. Tack weld the dome back into place at a few joints so as to keep it in place but also reduce its ability to transfer excess heat into the trailer directly through the steel parts. THEN...

foil the dome (to create a slip layer) and add 2-3 inches of refractory heat for thermal layer. I understand the steel dome itself will not cook great pizza. Then basically follow Wiley's steps with insulation blanket, rebar, chicken wire, finish with stucco.

[Wiley]

Re: Mobile oven design Buoy...HELP

Back to the main issue. For the life of me I can't see how you haven't painted yourself into a corner. Are you still planning to use the steel dome as your interior oven surface? Without any thermal mass? Sitting on what?

Your firebrick floor still touches the metal trailer housing? I work with metal a lot, and believe me, that stuff conducts heat.

I think you need to do some serious thinking at this point. If it were me, I'd be thinking in the direction of using your steel sphere as a waterproof covering to a real oven, with proper insulation all around. That sphere is immense by brick oven standards. You really don't need anything like that much interior cooking space for a portable pizza oven.

I concur,
Wiley

[dmun]

Re: Mobile oven design Buoy...HELP

Back to the main issue. For the life of me I can't see how you haven't painted yourself into a corner. Are you still planning to use the steel dome as your interior oven surface? Without any thermal mass? Sitting on what?

Your firebrick floor still touches the metal trailer housing? I work with metal a lot, and believe me, that stuff conducts heat.

I think you need to do some serious thinking at this point. If it were me, I'd be thinking in the direction of using your steel sphere as a waterproof covering to a real oven, with proper insulation all around. That sphere is immense by brick oven standards. You really don't need anything like that much interior cooking space for a portable pizza oven.

[RTflorida]

Re: Mobile oven design Buoy...HELP

Guys, my apologies if I insinuated you didn't have a clue what you were doing with the foil.
Foil seems to raise its ugly head every few months, its roots going back a long time with old school builders. From what I have read, their intention was a slip plane that would allow the layers to move independently, thus reducing movement of outer layers and reducing cracks. Problem is, even if the foil does not corrode, the slip plane would only work in one direction (like rubbing your palms together) and would do nothing for outward expansion.
In your application, stopping adhesion of the layers, I see no reason not to use it...foil is cheap.

RT

[Wiley]

Re: Mobile oven design Buoy...HELP

"I guess I assumed the foil was a temporary barrier anyway, to keep the thermal layer from bonding with the steel dome. What are the negative effects of losing the foil after the cement layer has cured?
Yesterday 10:41 PM"

Correct, the foil is simply acting as a "parting film". That it is corroded away is no problem as it is quite thin and so doesn't pose problems with expansion as it corrodes. I never made a case for any thermal increase nor for reduced cracking (I included it between the polygonal sections of refractory so that it would crack where I desired rather than randomly) . I included it in my build primarily to keep the calcium aluminate concrete from sticking to the steel dome during the construction and first firings. It was placed so that the two (refractory and steel dome) are in intimate contact yet not bonded. The intimate contact allows for rapid and efficient heat transfer from the steel dome to the refractory. The "not bonded" is so that stresses due to differences in expansion between the two materials can be accommodated.

Whether this same action is desirable in the construction of a more traditional brick domed WFO is for each to decide for themselves. Each builds their own WFO. When one diverges from the tried and true path (Forno Bravo Pompeii plans) one suffers or gains from their decisions.

Hope this helps,
Wiley

I disagree, friends are not over-rated. Sorry if your WFO has cost you a friendship, WFOs usually bring friends together.

[P712]

Re: Mobile oven design Buoy...HELP

I guess I assumed the foil was a temporary barrier anyway, to keep the thermal layer from bonding with the steel dome. What are the negative effects of losing the foil after the cement layer has cured?

[RTflorida]

Re: Mobile oven design Buoy...HELP

I'm going to side with dmun on the foil, it has been noted on many past threads that cement/concrete (portland cement) is highly corrosive to aluminum foil. In theory, a slip plane seems like a good idea, aluminum foil is not the answer. In no time the cement based product (vermiculite concrete) will be in direct contact with the steel dome, with the vermicrete layer moving with the steel. Many have said that foil is the reason for reduced cracking, science says that other variables are at play, its not the foil.

RT

[P712]

Re: Mobile oven design Buoy...HELP

sorry those photos are wrong side up

[P712]

Re: Mobile oven design Buoy...HELP

So update...had a smackdown with "friend" and it ended with me hooking up my trailer and drove off huffing. Oh well, friends are over rated. So I set up shop in my Dad's garage. Bought a torch and took the dome off at 5 inches up from weld. Managed to find some fire brick and rented a brick saw from HD for $50, it made fast work of those bricks. My plan is to lay down FB board from Forno Bravo store (2 inches) then lay the bricks on top (3 inches). After that I'm stumped as to connecting the dome back on while keeping it separated from the trailer some how.

[Tman1]

Re: Mobile oven design Buoy...HELP

More insulation in the floor will help, but I don't think that is the major issue. I'm thinking that you are correct and heat will go through the dome and into the trailer. You'd have to put a smaller dome INSIDE the buoy and insulate between the two.
I wonder if the perlite/vermiculite mix is strong enough that you could plaster it on the inside a for a couple inches? Then use insulation blanket, then a new dome? Certainly doesn't sound fun. Cutting the weld while you still can would probably be the best course of action. Although I know you don't want to hear that.

[P712]

Re: Mobile oven design Buoy...HELP

I appreciate the comments. The dome was welded after I gave instructions not to do so; the "friend" was trying to rush the job thinking he knew better. It does seem logical that the heat transfer will be a problem, even after the dome has its insulating layers. What about increasing the fire brick insulating layer from 3 inches up to 6 inches? Would that help keep heat in the oven and off the trailer? Or perhaps building a suspended poured masonry deck an inch or two off of the steel plate? I could weld a rebar frame into (but not attached to) the bottom 3 inches of the buoy steel plate using FB insulation board as the first one inch to keep the masonry deck elevated off the steel deck, then place fire brick deck, thus minimizing the heat transfer from the fire. Though I'm guessing much of the heat will be transferred from the flames rising up to the dome itself and transferring to the trailer.

Or recommendations on a better fix?

[Wiley]

Re: Mobile oven design Buoy...HELP

Wow!, I'm come late to this party...

First an observation:

When you started this thread you said you had a 48 inch diameter hemisphere and now it is a 57 1/2 inch hemisphere. If you search the archives you will find the posts on a WFO using a bouy. Some exchanges between that builder and myself are on my "Steel Dome Oven" thread. That WFO became "Dented Bouy" and is used commercially.

Dented Bouy used his bouy as the outer shell to his WFO. The interior dome was a 48 inch diameter steel hemisphere (ex propane tank). Outside that interior dome (in the interstitial space between the two shells) lie the refractory heatsink/reservoir and the insulation. The hearth bricks lie upon insulation and that insulation is supported by a steel deck.

Welding the inner dome to the base is IMHO not the way to proceed for the reasons stated by Dmun (and others). I am guessing that you welded the half dome to the plate in order to secure it and the finished WFO against movement during transport of the finished WFO.

I'm at a loss as to suggestions as to how to proceed from this point. Everything that I think of would be usurping your inventiveness and involve in cutting the dome off the trailer.... and I'm thinking such suggestions would not be welcomed.

As for Dmun's comment about foil being useless in this application (making sure there is no bond between the cement/fondu/refractory and the steel dome) I would have to say I hold a contrarian view. Steel and concrete/cement have a particularly good adhesive bond as those who have failed to completely wash a shovel used in placing concrete can attest. When applying the refractory to the exterior of the dome it is simply an extra layer that insures the refractory and steel dome are free to expand and contract independently as they heat and cool. An inexpensive bit of insurance.

Keep us posted as to your progress. I note that there were several months between your initial postings and the most recent with picture. That's unfortunate as perhaps the present circumstance could have been avoided.

Sorry, that I cannot at this point be more helpful,
Wiley

[Tman1]

Re: Mobile oven design Buoy...HELP

Yes, I think Dmun has a VERY valid point. Perhaps you have a solution you haven't shared.

[dmun]

Re: Mobile oven design Buoy...HELP

Forget the foil. It's completely useless.

I'm worried that with your dome welded to your support structure, that heat will zoom down to the trailer structure with alarming speed. No matter how you insulate inside the dome below the floor, or the exterior of the dome you may end up with something that's too hot to stand next to.

Vermiculite concrete may be too brittle for a mobile oven.

You may want to think about some thermal mass as well, as soon as you get that thermal bridge problem sorted.